Method for synchronizing synchronous data communication network and communication device used in the synchronous data communication network

ABSTRACT

A method of switching from one synchronizing signal source to another in a synchronous data communication network in which a plurality of stations are connected to each other via lines and A plurality of synchronizing signal sources are provided inside or outside the stations; the when a failure occurs a synchronizing signal source is replaced by another synchronizing signal source in correspondence to the failure so that the data communication may be continued. Specifically, flag bit data, indicating whether or not the synchronizing signal transmitted via the line is available, are set in the signal including the synchronizing signal and transmitted via the line. When a failure occurs in any of the plurality of synchronizing signal sources, a table provided in each station and specifying the order of priority for selection of a synchronizing signal source is referred to, and a synchronizing signal source is selected for each station in accordance with said flag bit data. Each station switches from the currently selected synchronizing signal source to the synchronizing signal source selected.

FIELD OF THE INVENTION

The present invention relates to a synchronous data communicationnetwork, and more particularly to a method of switching among aplurality of synchronizing signal sources.

BACKGROUND ART

Recently, as a consequence of a demand for standardization of digitalcommunication networks, many of those networks are synchronous networks,and reliability of a synchronizing signal responsible forsynchronization is called for. Known synchronous communication networksinclude the one using an optical fiber cable for performingtransmissions of high-speed digital signals. In such a synchronouscommunication network, an oscillator for generating a main clock isprovided in a system. This main clock is shared by both the transmittingside and the receiving side. Normally, a plurality of input signals, bybeing subjected to a plurality of times of hierarchical multiplexingprocesses, are converted into a high-speed multiplexed signals beforetransmission. The input signals are multiplexed in a byte unit. Uponeach multiplexing, signal transmission rate increases.

One of the known high-speed transmission network utilizingbyte-multiplexing is SONET (synchronous optical network). As shown inFIG. 1, an STS-1 signal of this SONET system is configured such that oneframe is constructed of 6480 bits (=90 bytes× 8 rows×8 bits), where 1byte represents 8 bits. The duration of one frame is 125 μs, and the bitrate is 51.84 MHz. The frame format of the STS-1 signal shown in FIG. 1is provided in each channel. The headmost 2 bytes of the frame formatare frame synchronizing patterns A1, A2, and the next 1 byte is achannel identification pattern C1. SOH (section overhead), LOH (lineoverhead), and POH (path overhead) are control data added to theinformation to be transmitted.

A plurality of STS-1 signals having the above frame format are subjectedto a simple byte-multiplexing (meaning that no format conversion iscarried out). FIG. 2 shows how three STS-1 signals are byte-multiplexed.The STS-1 signals for three S channels #1, #2 and #3 arebyte-multiplexed so that an STS-3 signal having 155.52 MHz rate isgenerated. This STS-3 signal is standardized as an STM-1 signalaccording to the CCITT recommendation. It is assumed that an STS-1signal is transmitted as an optical signal. At the head of the data inthe three channels #1-#3, the 2-byte frame synchronizing patterns A1, A2and the 1-byte channel identification pattern C1 are added. As indicatedby broken lines, the STS-3 signal is formed by byte-multiplexing. It isto be noted that no frame patterns unique to the resultant STS-3 signalare inserted. Byte-multiplexing in this system is carried out such thatthe heads of the signals on the channels #1-#3 are in sync with eachother, with the result that the frame-multiplexed synchronizing patternof an STS-3 signal is of a 6-bit construction.

The frame synchronizing patterns A1, A2 are the same for the channels#1-#3, A1 being "11110110" and A2 being "00101000". The channelidentification patterns C1 for the channels #1-#3 are set to bedifferent from channel to channel.

Referring back to FIG. 1, B1-B3 are byte interleave parities; C2 is asignal label byte indicative of the presence or absence of information;D1-D12 are data communication bytes for transporting, for example,information relating to the state of different units; E1, E2 are orderwire bytes; F1, F2 are user channel bytes; G1 is a path status byte fordetecting a parity error of an input signal and returning the detectederror to the originating unit; H1, H2 are pointers having a stuffingfunction for incorporating an asynchronous system; H3 is a pointerhaving a variable slot function in stuffing; H4 is a multi-frameindication byte; J1 is a trace byte; K1, K2 are automatic protectionswitch bytes; and Z1-Z5 are reserved bytes.

At the receiving side, frame synchronization is effected by detectingthe 6-byte frame multiplexed synchronizing pattern of the STS-3 signalas shown in FIG. 2. As indicated by the broken lines, the signal isdivided into the data for each of the channels #1-#3, whereupon thechannel identification pattern C1 is utilized to determine whether theaccurate separation is achieved.

It is possible to further multiplex the STS-1 signal. In a mannersimilar to the above, the frame synchronizing patterns A1, A2 and thechannel identification pattern C1 at the head of the STS-1 signal arebyte multiplexed and provided at the head of the STS-n signal obtainedas a result of n-multiplexing. The frame multiplexed synchronizingpatterns provided at the head of the STS-n signal is of a 2 n byteconstruction.

As has been described, the conventional synchronizing data communicationnetwork allows only one synchronizing signal source in the network andis merely capable of informing to the outside that a failure hasoccurred and the timing fails to be maintained. Accordingly, datacommunication is disabled when a failure occurs and prevents the timingto be maintained.

DISCLOSURE OF THE INVENTION

An object of the present invention is to eliminate the abovedisadvantage.

More specifically, an object of the present invention is to provide asynchronizing method of a synchronous data communication network inwhich a plurality of synchronizing signal sources are provided in a datacommunication network and data communication is maintained upon anoccurrence of a failure by efficiently switching among the synchronizingsignal sources in response to the failure that occurred, as well as toprovide a communication apparatus used in such a synchronous datacommunication network.

The object of the present invention can be achieved by a method ofswitching from one synchronizing signal source to another in asynchronous data communication network comprising: a plurality ofstations connected to each other via lines; and a plurality ofsynchronizing signal sources provided inside or outside the stations,the method comprising steps of: (a) setting, in a signal transmitted viathe line and including the synchronizing signal, flag bit data (S, *S)indicating whether or not the synchronizing signal transmitted via thelines is available; (b) allowing each station to refer, on the basis ofthe flag bit data, to a table provided in each station and specifyingthe order of priority for selection of a synchronizing signal source,upon occurrence of a failure in any of the plurality of synchronizingsignal sources, and selecting a synchronizing signal source; and (c)allowing each station to switch from the currently selectedsynchronizing signal source to the synchronizing signal source selectedin the step (b).

The object of the present invention can also be achieved by acommunication apparatus used in a synchronous data communication networkhaving a plurality of signal sources, the apparatus comprising: firstmeans for receiving flag bit data (S, *S) indicating whether or not asynchronizing signal transmitted via a line is available, whichsynchronizing signal is included in the signal transmitted via thelines; second means for specifying the order of priority for selectionof a synchronizing signal source; third means for referring, uponoccurrence of a failure in any of the plurality of synchronizing signalsources, to the order of priority for selection of a synchronizingsignal source, which order is specified by the second means, and forselecting, on the basis of the flag bit data, a synchronizing signalsource; and fourth means for sending, in correspondence to thesynchronizing signal source selected by the third means, a flag bitdata, indicating whether or not the synchronizing signal from thecommunication apparatus originating the flag bit is available, to aneighboring communication apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an example of a synchronous datacommunication network;

FIG. 2 illustrates a signal multiplexing operation in the synchronousdata communication network shown in FIG. 1;

FIG. 3 illustrates a normal operation of the first embodiment of thepresent invention;

FIG. 4 illustrates the first stage of a failure occurring in the firstembodiment of the present invention;

FIG. 5 illustrates the second stage of a failure occurring in the firstembodiment of the present invention;

FIG. 6 illustrates the third stage of a failure occurring in the firstembodiment of the present invention;

FIG. 7 illustrates the fourth stage of a failure occurring in the firstembodiment of the present invention;

FIG. 8 illustrates the fifth stage of a failure occurring in the firstembodiment of the present invention;

FIG. 9 illustrates the sixth stage of a failure occurring in the firstembodiment of the present invention;

FIG. 10 illustrates the seventh stage of a failure occurring in thefirst embodiment of the present invention;

FIG. 11 illustrates the first stage of a recovery in the firstembodiment of the present invention;

FIG. 12 illustrates the second stage of a recovery in the firstembodiment of the present invention;

FIG. 13 illustrates the third stage of a recovery in the firstembodiment of the present invention;

FIG. 14 illustrates the fourth stage of a recovery in the firstembodiment of the present invention;

FIG. 15 illustrates the first stage of a failure occurring in the secondembodiment of the present invention;

FIG. 16 illustrates the second stage of a failure occurring in thesecond embodiment of the present invention;

FIG. 17 illustrates the third stage of a failure occurring in the secondembodiment of the present invention;

FIG. 18 illustrates the first stage of a recovery in the secondembodiment of the present invention;

FIG. 19 illustrates the second stage of a recovery in the secondembodiment of the present invention;

FIG. 20 is a block diagram illustrating the configuration of a station;

FIG. 21 shows determination operations in the stations in a linear modearrangement;

FIG. 22 illustrates a normal state of the third embodiment of thepresent invention;

FIG. 23 illustrates the first stage of a failure occurring in the thirdembodiment of the present invention;

FIG. 24 illustrates the second stage of a failure occurring in the thirdembodiment of the present invention;

FIG. 25 illustrates the third stage of a failure occurring in the thirdembodiment of the present invention;

FIG. 26 illustrates the first stage of a recovery of the thirdembodiment of the present invention;

FIG. 27 illustrates the second stage of a recovery of the thirdembodiment of the present invention;

FIG. 28 illustrates a normal state of the fourth embodiment of thepresent invention;

FIG. 29 illustrates the first stage of a failure occurring in the fourthembodiment of the present invention;

FIG. 30 illustrates the second stage of a failure occurring in thefourth embodiment of the present invention;

FIG. 31 illustrates the third stage of a failure occurring in the fourthembodiment of the present invention;

FIG. 32 illustrates the fourth stage of a failure occurring in thefourth embodiment of the present invention;

FIG. 33 illustrates the fifth stage of a failure occurring in the fourthembodiment of the present invention;

FIG. 34 illustrates the first stage of a recovery in the fourthembodiment of the present invention;

FIG. 35 illustrates the second stage of a recovery in the fourthembodiment of the present invention;

FIG. 36 illustrates the third stage of a recovery in the fourthembodiment of the present invention;

FIG. 37 illustrates the first stage of a failure occurring in the fifthembodiment of the present invention;

FIG. 38 illustrates the second stage of a failure occurring in the fifthembodiment of the present invention;

FIG. 39 illustrates the third stage of a failure-occurring in the fifthembodiment of the present invention;

FIG. 40 illustrates the fourth stage of a failure occurring in the fifthembodiment of the present invention;

FIG. 41 illustrates the first stage of a recovery in the fifthembodiment of the present invention;

FIG. 42 illustrates the second stage of a recovery in the fifthembodiment of the present invention; and

FIG. 43 shows determination operations in the stations arranged in aring mode.

BEST MODE OF CARRYING OUT THE INVENTION

The synchronizing signal source switching method of the presentinvention, as applied to a digital communication network having aplurality of stations A, B, C, and D connected via lines and comprisinga plurality of synchronizing signal sources, allows the following datato be set in an overhead bit of a signal transmitted to the stations,i.e.: a synchronizing bit data, by which a synchronizing signal istransmitted; and flag bit data S, *S indicating whether or not thesynchronizing signal is included in the transmitted signal and isavailable. Additionally, the stations are provided with tables T-A, T-B,T-C and T-D specifying the order of priority for selection of asynchronizing signal source in the stations. The order of priority issuch that, for a parent station A using an external synchronizing signalsource during a normal operation, the external synchronizing signalsource is given the highest order, while an internal signal source isgiven the lowest order; and, for a parent station using an internalsynchronizing signal source INT during a normal operation, the internalsynchronizing signal source INT is given the highest order. As regardschild stations B, C and D receiving the synchronizing signal from otherstations, the station from which they receive the signal during a normaloperation is given the highest order, followed by the station from whichthey receive the synchronizing signal when a failure occurs. In the casethat the child station has an internal synchronizing signal source INT,the internal synchronizing signal source INT is given the lowest order.The synchronizing signal sources in the network include a plurality ofexternal synchronizing signal sources EXT and the internal synchronizingsignal sources INT provided in the stations. The stations in thiscommunication network may be in a linear mode arrangement, a ring modearrangement or in an arrangement which is a combination of these twomodes of arrangement.

The present invention allows the parent station A to send, during anormal operation, a flag bit data S, indicating that the synchronizingsignal thereof is available, to all of its neighboring stations, and tosend, when there is a failure in the synchronizing signal source, a flagbit data *S indicating that the same signal is not available. When theflag bit data from any of the neighboring stations indicate theavailability of the synchronizing signal source, the child stations B,C, D selects, from among the neighboring stations having their flag bitdata indicating that their synchronizing signal is available, a stationto receive the synchronizing signal from, the selection being based onthe order of priority specified in the table. The child stations thensends the flag bit data, indicating that the synchronizing signaltherefrom is not available, to the selected station, while at the sametime sending the flag bit data, indicating that the same signal isavailable, to the other station connected thereto. When the flag bitdata from a neighboring station connected to the child station indicatesnon-availability of the synchronizing signal source, such a neighboringstation having its flag bit data indicating non-availability isneglected in the process of selection based on the order of priority. Achild station operated by using the internal synchronizing signal sourceINT sends the flag bit data indicating non-availability to theneighboring stations.

Both during the normal operation and when a failure occurs, each stationchecks the flag bit data of the synchronizing signal sources constantlysearching for a source having higher priority. Specifically, thestations successively carry out, in a manner resembling a chainreaction, a process of selecting a synchronizing signal source inaccordance with the above-described procedure which involves a processof referring to the specified order of priority, and then changing, onthe basis of that selection, the transmitted flag bit data. When thefailure is removed, the same chanin reaction of selecting and changingis successively carried out by the stations in accordance with theprocedure already mentioned, until the normal state is regained. Whilethe internal synchronizing signal source in the child station may betemporarily employed while the above process is successively carried outfollowing an occurrence of a failure, the stations are finally put insynchronous operation using one or two synchronizing signal sources. Thesame thing applies to a process of recovering from a failure.

The internal synchronizing signal sources of the stations, which sourcesare employed temporarily, are in a stand-by status by maintaining itssynchronousness with the synchronous signal in the communicationnetwork. Since the switching, from a synchronizing signal sourceexternal to the station to the internal synchronizing signal source, isconducted in an extremely short time, the phase shift of thesynchronizing signals thereof can be neglected, thus enabling ahigh-precision high-reliability synchronizing signal switching.

A synchronizing bit data for transmitting the synchronizing signal and aflag bit data indicating availability/non-availability of thesynchronizing signal are provided in a predetermined position of theoverhead of the signal transmitted through the stations in the digitalcommunication network of the present invention. For example, theaforementioned reserved byte Z1 may be used. In the embodiments thatfollow, S signifies a bit data indicating availability of asynchronizing signal source, and *S signifies a bit data indicatingnon-availability.

A detailed description will be given below of the first embodiment ofthe present invention, in which the stations A through D are seriallyarranged, that is, are in a linear mode arrangement, and two externalsignal sources EXT (P) and EXT (S) are connected to the stations A and Dat both ends of the series, respectively. The first externalsynchronizing signal source EXT (P) is connected to the station A as amain signal source. Accordingly, the station A will be called a parentstation. The second external synchronizing signal source EXT (S) isconnected to the station D as a reserved signal source. Symbols E and Wdenote directions, E signifying the right hand side and W signifying theleft hand side. Referring to the figures, a solid line signifies a flowof a synchronizing signal and a flag bit data, and a broken linesignifies a flow of a flag bit data as it is processed in each stationaccording to the rule specified below. Squares T-A, T-B, T-C and T-D aretables showing the order of priority preset in each station forselection of one of the synchronizing signal sources. The source enteredat the top of the table is given the highest priority. Therefore, thesynchronizing signal source used during a normal operation is given thehighest order, followed by the source in the neighboring station whichsource should be selected when a failure occurs. If the station has aninternal synchronizing signal source INT, it is given the lowest order.In the tables, a solid black circle signifies the currently used signalsource, an X signifies that the signal source is not available even ifit is requested, and a Circle signifies that the source is available atthat moment.

FIG. 3 shows a normal operation. A description will be given below of aprocedure by which the synchronizing signal transmission channel foreach station is switched when the first external synchronizing signalsource EXT (P) is disconnected. FIG. 4 illustrates the first stage of afailure. The station A detects the disconnection of the externalsynchronizing signal source EXT (P) and checks the table T-A to see ifthe line connected to the station A and given the next highest order,that is, the station B in this case, is available. Since, however, theflag bit data supplied to that line is *S indicating non-availability,the internal synchronizing signal source INT, having the next highestorder of priority is selected. At the same time as this, the flag bitdata S, which indicates the availability of the synchronous signal andhad been sent to the line connected to the station B, is changed to theflag bit data *S.

FIG. 5 illustrates the second stage of a failure that follows the stageshown in FIG. 4. The station B, which has received the flag bit data *Sfrom the adjacent station A to the E direction, refers to the order ofpriority specified in the table T-B and checks the station C to the Wdirection. Since, however, the flag bit data *S is detected again, thestation B selects the internal synchronizing signal source INT andchanges the flag bit data supplied to the station C, from S to *S.

FIG. 6 illustrates the third stage that follows the stage shown in FIG.5. The station C selects the internal synchronizing signal source byfollowing steps similar to the ones taken by the station B at the secondstage described above, while at the same time changing the flag bit datasupplied to the station D, from S to *S.

FIG. 7 illustrates the fourth stage that follows the stage shown in FIG.6. The station D detects *S on the line connected to the station C,selects the second external synchronizing signal source EXT (S) havingthe next highest priority according to the table T-D, while at the sametime changing the flag bit data supplied to the line connected to thestation c, from *S to S.

FIG. 8 illustrates the fifth stage that follows the stage shown in FIG.7. The station C, which checks the state of the synchronizing signalsources constantly searching for a source having higher priority,checks, in accordance with the order of priority specified in the tableT-C, the line to the E direction and detects *S. The station C thenchecks the line connected to the station D to the W direction, whichline has the next highest priority, and, finding that S is senttherefrom, changes the flag bit data supplied to the line connected tothe station B, from *S to S.

FIG. 10 illustrates the seventh stage that follows the stage shown inFIG. 9. The station A detects S on the line connected to the station B,which line is then selected, while at the same cancels the use of theinternal synchronizing signal source INT having lower priority than thestation B. A stable state now being attained, all of the stationsoperate in sync with the second external synchronizing signal source EXT(S).

A description will be given below of the procedure for switching, whenthe system recovers from the failure in the first external synchronizingsignal source EXT (P), the channel on which the synchronizing signal istransmitted to each station.

FIG. 11 illustrates the first stage of a recovery. The station A detectsthe synchronizing signal from the first external synchronizing signalsource EXT (P) and cancel the selection of the synchronizing signal fromthe station B having lower priority than the external source EXT (P)according to the table T-A, while at the same time changing the flag bitdata supplied to the station B, from *S to S.

FIG. 12 illustrates the second stage of a recovery that follows thestage shown in FIG. 11. The station B detects S in a transmission viathe line connected to the station A, which line is then selected, andcancels the selection of the synchronizing signal from the station Cbecause the line connected thereto has lower priority than thatconnected to the station A, while at the same time changing the flag bitdata supplied to the station C, from *S to S.

FIG. 13 illustrates the third stage of a recovery that follows the stageshown in FIG. 12. The station C follows the same steps as those followedby the station B in the second stage of a recovery, selects the lineconnected to the station B after detecting S in that line, and cancelsthe selection of the synchronizing signal from the station D because theline connected thereto has lower priority than that connected to thestation B, while at the same time changing the flag bit data supplied tothe station D, from *S to S.

FIG. 14 illustrates the fourth stage of recovery that follows the stageshown in FIG. 13. The station D detects S on the line connected to thestation C, which line is then selected, and cancels the selection of theexternal synchronizing signal having lower priority than the station C,while at the same time changing the flag bit data supplied to thestation C, from *S to S. This way, the normal state shown in FIG. 3 isregained.

A description will next be given of the second embodiment in which afailure in the transmission circuit is assumed to occur between thestation B and the station C under the same configuration as in the firstembodiment, with reference to FIGS. 15 through 19. FIG. 15 illustratesthe first stage of a failure where the station B and the station C aredisconnected. The station C, which had received the synchronizing signalfrom the station B, now detects an absence of the flag bit data S in thereceived signal, checks the line to the W direction, which line has thenext highest priority according to the table T-C. Since, however, theflag bit data supplied to the line is *S indicating non-availability ofthe synchronizing signal, the internal synchronizing signal source INThaving the next highest priority is selected, while at the same time theflag bit data that had been transmitted to the line connected to thestation D is changed from S to *S.

FIG. 16 illustrates the second stage that follows the stage shown inFIG. 15. The station D detects *S in a transmission via the lineconnected to the station C and selects the second external synchronizingsignal source EXT (S) having the next highest priority according to thetable T-D, while at the same time changing the flag bit data supplied tothe line connected to the station C, from *S to S.

FIG. 17 illustrates the third stage that follows the stage shown in FIG.16. The station C, which checks the state of the synchronizing signalsources constantly searching for a source having higher priority,detects S on the line connected to the station D to the W direction,which line has higher priority than the internal synchronizing signalsource INT according to the table T-C. The station C then selects theline connected to the station D, while at the same time changing theflag bit data supplied to the line connected to the station B, from *Sto S. This changing of the flag bit data is carried out merely by way ofobserving the rule, and the flag S does not actually reach the stationB. A stable state thus being attained, the first external synchronizingsignal source EXT (P) temporarily holds the station A and the station B,that is the stations to the E direction as viewed from the point ofoccurrence of a failure. The second external synchronizing signal sourceEXT (S) holds the stations C and D, that is, the stations to the Wdirection, in sync with each other.

A description will next be given of the procedure for recovering, whenthe failure is removed, the channel for transmitting the synchronizingsignal to the stations.

FIG. 18 illustrates the first stage of a recovery. The station C detectsS on the line connected to the station B, selects the same station inaccordance with the table T-C, and cancels the selection of thesynchronizing signal from the station D because the line connectedthereto has lower priority than the line connected to the station B,while at the same time changing the flag bit data supplied to thestation D, from *S to S and changing the flag bit data supplied to thestation B, from S to *S.

FIG. 19 illustrates the second stage of recovery that follows the stageshown in FIG. 18. The station D detects S on the line connected to thestation C, selects the same station in accordance with the table T-C,and cancels the selection of the external synchronizing signal sourceEXT (S) having lower priority than the station C, while at the same timechanging the flag bit data supplied to the station C, from S to *S. Thisway, the initial normal state shown in FIG. 15 is regained.

The configuration of the stations A-D will next be described, withreference to FIG. 20. The configuration of the station shown in FIG. 20pertain directly to the stations B and C. That is, FIG. 20 shows theconfiguration of a station connected to the lines on both sides, namely,the side to the E direction and the side to the W direction. Such astation (indicated by a numeral 100 in the figure) comprises adisconnection detection portion 10, a selection switch 12, a firstdetermination portion 14, a second determination portion 16, ademultiplexor (DMUX) portion 18, a multiplexor (MUX) portion 20, amultiplexor (MUX) portion 22, a demultiplexor (DMUX) portion 24 and aninternal synchronizing signal source (INT) 26. The station A is notprovided with the demultiplexor 18 and the multiplexor 20. The station Dis not provided with the multiplexor 22 and the demultiplexor 24.

The disconnection detection portion 10 receives the synchronizing signaland detects the disconnection thereof. The disconnection detectionportion 10 of the above-mentioned station A receives the synchronizingsignal from the external synchronizing signal source EXT (P), thesynchronizing signal from the internal synchronizing signal source 26and the synchronizing signal supplied from the neighboring station viathe line. Similarly, the disconnection detection portion 10 of thestation D receives the synchronizing signal from the externalsynchronizing signal source EXT (S), the synchronizing signal from theinternal synchronizing signal source 26 and the synchronizing signalsupplied from the neighboring station via the line. The disconnectionportions 10 of the stations Band. C receive the synchronizing signalfrom the internal synchronizing signal source 26 and the synchronizingsignal supplied from the neighboring stations via the line to the Edirection and the line to the W direction. For example, thedisconnection portion 10 may be equipped with internal timers for eachdifferent synchronizing signal received and gives a determination of anabsence of the synchronizing signal if that synchronizing signal is notreceived for a predetermined period of time.

When the disconnection detection portion 10 detects a disconnection, itoutputs disconnection information to the first determination portion 14.Upon receiving this disconnection information, the first determinationportion 14 gives a determination described later, by using the receivedflag bit data (S, *S) from the demultiplexor 18 and the received flagbit data (S, *S) from the demultiplexor 24. The first determinationportion 14 outputs a control signal corresponding to the determinationresult to the selection switch 12. The selection switch 12 selects, onthe basis of the control signal, one of the synchronizing signals andoutputs, as the transmitted synchronizing signal, the selected signal tothe multiplexors 20 and 22. The selection switch 12 outputs theselection information, indicating which synchronizing signal has beenselected, to the second determination portion 16. The seconddetermination portion 16 gives a determination described later by usingthe selection information received and sets the transmitted flag bitdata. In accordance with the determination result, the seconddetermination portion 16 then outputs the transmitted flag bit data (S,*S) to the multiplexor 20 and outputs the transmitted flag bit data (S,*S) to the multiplexor 22.

The demultiplexor 18 separates, into frames, the frame multiplexed linesignal upon receiving the same signal via the line to the E directionand sends the result of separation to a signal processing portion (notshown in the figure). Moreover, the demultiplexor 18 extracts thesynchronizing signal and the received flag bit data. The multiplexor 20multiplexes a frame supplied from the signal processing portion (notshown in the figure), the transmitted synchronizing signal selected bythe selection switch 12 and the transmitted flag bit data set by thesecond determination portion 16, and sends the resultant line signal tothe line to the E direction. The multiplexor 22 multiplexes a framesupplied from the signal processing portion (not shown in the figure),the transmitted synchronizing signal selected by the selection switch 12and the transmitted flag bit data set by the second determinationportion 16, and sends the resultant line signal to the line to the Wdirection. The demultiplexor 24 separates, into frames, the framemultiplexed line signal received via the line to the W direction, andsends the separation result to the signal processing portion (not shownin the figure). Moreover, the demultiplexor 22 extracts thesynchronizing signal and the received flag bit data.

FIG. 21 illustrates the stations A-D in a linear mode arrangement andshows the determination processes 1 and 2 carried out by thedetermination portion 14 and the determination portion 16 in thestations. In the figure, "◯" indicates a normal operation (where nodisconnection takes place), and "X" indicates the detection of adisconnection. P1, P2 and P3 indicate the order of priority forselection of a synchronizing signal source. Holdover means a runawaystate.

For example, the determination process of the first determinationportion 14 of the station A is as follows. The order of priority forselection of a synchronizing signal source is such that the highestpriority is given to the external synchronizing signal source EXT (P)followed by the line and the internal synchronizing signal source, inthe stated order. When the external synchronizing signal source becomesdisabled, the synchronizing signal from the line becomes a candidate fora synchronizing signal source. If, at this moment, the received flag bitdata is S, the line is selected. If the received flag bit data is *S,the internal synchronizing signal source 26 is selected. It is of courseassumed here that the internal synchronizing signal source 26 isoperating normally. If the internal synchronizing signal source 26 isnot operating normally, a runaway state sets in. When both the externalsynchronizing signal source EXT (P) and the line are disabled but theinternal synchronizing signal source 26 is operating normally, theinternal synchronizing signal source 26 is selected. When all thesynchronizing signal sources are disabled, a runaway state sets in.

The determination process of the second determination portion 16 of thestation A is as follows. When the external synchronizing signal sourceEXT (P) is selected, the transmitted flag bit data is set to be S. Whenthe line or the internal synchronizing signal source 26 is selected, orwhen a runaway state has set in, the transmitted flag bit data is set tobe *S.

The first determination portion 14 and the second determination portion16 of the stations B and C operate as shown in the figure. The order ofpriority of the synchronizing signal sources for the stations B and C issuch that the highest priority is given to the line to the E direction,followed by the line to the W direction and the internal synchronizingsignal source 26, in the stated order. When the line to the E directionis operating normally and the received flag bit data supplied from theline to the E direction is S, the line to the E direction is selected asthe synchronizing signal source. When the received flag bit datasupplied from the line to the E direction is *S and the received flagbit data supplied from the line to the W direction is S, the line to theW direction is selected. When the received flag bit data supplied fromthe line to the E direction and the line to the W direction are *S and*S, respectively, under the condition that the internal synchronizingsignal source 26 is operating normally, the internal synchronizingsignal source 26 is selected. When the line to the E direction isoperating normally but the internal synchronizing signal source 26 isdisconnected, a holdover sets in. When the received flag bit datasupplied from the line to the W direction is S under the condition thatthe line to the E direction is disconnected and the line to the Wdirection is operating normally, the line to the W direction is selectedas the synchronizing signal source. When, in this case, theabove-mentioned received flag bit data is *S, the internal synchronizingsignal source 26 is selected. When the received flag bit data suppliedfrom the line to the W direction is *S under the condition that only theline to the W direction is operating normally, a holdover sets in. Whenall of the lines are disconnected and only the internal synchronizingsignal source 26 is operating normally, the internal synchronizingsignal source 26 is selected. When all of the synchronizing signalsources are disconnected, a holdover sets in.

The second determination portion 16 of the stations B and C operates asfollows. When the line to the E direction is selected, the transmittedflag bit data supplied to the line to the E direction and the line tothe W direction are set to be *S and S, respectively. When the line tothe W direction is selected, the transmitted flag bit data supplied tothe line to the E direction and the line to the W direction are set tobe S and *S, respectively. When the internal synchronizing signal source26 is selected or when a holdover sets in, the transmitted flag bit datasupplied to the line to the E direction and the line to the W directionare set to be *S and *S, respectively.

The first determination portion 14 and the second determination portion16 of the station D operate as shown in FIG. 21. The order of priorityfor selection of a synchronizing signal source for the station D is suchthat the highest priority is given to the line, followed by the externalsynchronizing signal source EXT (P) and the internal synchronizingsignal source 26, in the stated order.

A detailed description will next be given of the third embodiment, inwhich the stations A through D are arranged in a ring, that is, are in aring mode arrangement, and an external synchronizing signal source EXTis connected to the station A. For this reason, the station A isdesignated a parent station. FIG. 22 shows a normal state of the thirdembodiment, where the signal from the external synchronizing signalsource EXT is transmitted from the parent station A to the station B inthe W direction in the figure. The same signal is transmitted from thestation D to the station C in the E direction in the figure.

FIG. 23 illustrates the first stage of a disconnection of transmissionbetween the station A and the station D. The station which had receivedthe synchronizing signal from the station A now detects an absence ofthe flag bit data S in the received signal and checks the line connectedto the station C to the E direction, which line has the next highestpriority according to the table T-D. Since, however, the flag bit datasupplied from the line is *S, the internal synchronizing signal sourcehaving the next highest priority is selected, while at the same time theflag bit that had been supplied to the line connected to the station Cis switched from S to *S.

FIG. 24 illustrates the second stage that follows the stage shown inFIG. 23. The station C detects *S on the line connected to the station Dand checks the line to the E direction having the next highest priority,that is, the line connected to the station B. Since the flag bit datasupplied therefrom is S, the station B is put to use as a synchronizingsignal source, and the flag bit data supplied to the line connected tothe station B is changed from S to *S.

FIG. 25 illustrates the third stage that follows the stage shown in FIG.24. The station D, which checks the state of the synchronizing signalsources constantly searching for a source having higher priority,detects S on the line connected to the station C to the E direction,which line has higher priority than the internal synchronizing signalsource INT according to the table T-D, selects the line connected to thestation C, and cancels the selection of the internal synchronizingsignal source INT because the line connected thereto has lower prioritythan the line connected to the station C, while at the same timechanging flag bit supplied to the line connected to the station A, from*S to S. A stable state thus being attained, the synchronizing signalfrom the parent station A is temporarily transmitted through the childstations B, C, D in the W direction.

A description will next be given of the procedure for recovering thechannel by which to transmit the synchronizing signal to the stations.

FIG. 26 illustrates the first stage of a recovery. The station D detectsS on the line connected to the station A to the W direction, selects thesame station in accordance with the table T-D and cancels the selectionof the synchronizing signal supplied from the station C to the Edirection because the line connected thereto has lower priority than theline connected to the station A, while at the same time changing theflag bit data supplied to the station A, from S to *S, and changing theflag bit data supplied to the station C, from *S to S.

FIG. 27 illustrates the second stage of recovery that follows the stageshown in FIG. 26. The station C, which checks the state of thesynchronizing signal sources constantly searching for a source havinghigher priority, detects S on the line connected to the station D to theW direction, selects the same line in accordance with the table T-C, andcancels the reception of the synchronizing signal from the station B tothe E direction because the line connected thereto has lower prioritythan the line connected to the station D, while at the same timechanging the flag bit data supplied to the station D, from S to *S, andchanging the flag bit data supplied to the station B, from *S to S. Thisway, the initial normal state shown in FIG. 18 is regained.

A description will next be given, with reference to FIGS.28 through 36,of the fourth embodiment, in which the stations A through D are in aring mode arrangement as in the third embodiment and the externalsynchronizing signal source is connected to the station A, but thesynchronizing signal is made to circulate in the E direction withrespect to the parent station, that is, all of the child stations Bthrough D are fed the synchronizing signal from the W direction thereof.Unlike the third embodiment, such an operation requires that all of thetables specifying the order of priority for the child stations allow theline (W) to be placed at the top of the order, as shown in FIG. 28.

FIG. 29 illustrates the first stage of a disconnection of transmissionbetween the station A and the station D. As in the third embodiment, thestation D, which had received the synchronizing signal from the stationA, detects an absence of the flag bit data S and checks the lineconnected to the station C to the E direction, which line has the nexthighest priority according to the table T-A. Since, however, the flagbit supplied from that line is *S, the internal synchronizing signalsource INT having the next highest priority is selected, while at thesame time the flag bit data that had been supplied to the line connectedto the station C is changed from S to *S.

FIG. 30 illustrates the second stage that follows the stage shown inFIG. 29. The station C detects *S on the line connected to the station Dand checks, in accordance with the table T-B, the line connected to thestation B to the E direction, which line has the next highest priority.Since *S is again detected, the internal synchronizing signal source INTis put to use, while at the same time the flag bit data supplied to theline connected to the station B is changed from S to *S.

FIG. 31 illustrates the third stage that follows the stage shown in FIG.30. Detecting *S on the line connected to the station C to the Wdirection and having the highest priority according to the table T-B,the station B checks the flag bit data supplied from the line connectedto the station A to the E direction, which line has the next highestpriority. Since the flag bit data S is detected, the station A isselected as the synchronizing signal source, while at the same time theflag bit data supplied to the line connected to the station A is changedfrom S to *S, and the flag bit data supplied to the line connected tothe station C is changed from *S to S.

FIG. 32 illustrates the fourth stage that follows the stage shown inFIG. 31. The station C, which checks the state of the synchronizingsignal sources constantly searching for a source having higher priority,detects S on the line connected to the station B and changes, inaccordance with the table T-C, over to the line connected to the stationB as the synchronizing signal source, while at the same time changingthe flag bit data supplied to the station D, from *S to S.

FIG. 32 illustrates the fourth stage that follows the stage shown inFIG. 31. The station C, which checks the state of the synchronizingsignal sources constantly searching for a source having higher priority,detects S on the line connected to the station to the E direction, whichline has higher priority than the internal synchronizing signal sourceINT according to the table T-D, and selects the same line, while at thesame time changing the flag bit data supplied to the line connected tothe station A, from *S to S. A stable state thus being attained, thesynchronizing signal is temporarily transmitted through the childstations B, C, D in the W direction with respect to the parent stationA.

A description will below be given of how the channel for transmittingthe synchronizing signal to the stations is recovered when the failureis removed.

FIG. 34 illustrates the first stage of a recovery. The station D detectsS on the line connected to the station A to the W direction, selects thesame line in accordance with the table T-D and cancels the selection ofthe synchronizing signal from the station C because the line connectedthereto has lower priority than the line connected to the station A,while at the same time changing the flag bit data supplied to thestation A, from S to *S, and changing the flag bit data supplied to thestation C, from *S to S.

FIG. 35 illustrates the second stage of a recovery that follows thestage shown in FIG. 34. The station C, which checks the state of thesynchronizing signal sources constantly searching for a source havinghigher priority, detects S on the line connected to the station D to theW direction, selects the same line in accordance with the table T-C andcancels the selection of the synchronizing signal from the station B tothe E direction because the line connected thereto has lower prioritythan the line connected to the station D, while at the same timechanging the flag bit data supplied to the station D, from S to *S, andchanging the flag bit data supplied to the station B, from *S to S.

FIG. 36 illustrates the third stage of a recovery that follows the stageshown in FIG. 35. The station B, which constantly checks the state ofthe synchronizing signal sources having higher priority than thecurrently used source, detects S on the line connected to the station Cto the W direction and selects the same line in accordance with thetable T-B and cancels the reception of the synchronizing signal from thestation A to the E direction because the line connected thereto haslower priority than the line connected to the station C, while at thesame time changing the flag bit supplied to the station C, from S to *S,and changing the flag bit data supplied to the station A, from *S to S.This way, the initial normal state shown in FIG. 24 is regained.

A description will below be given of the fifth embodiment, in which thestations A through D are in a ring mode arrangement as in the thirdembodiment, the external synchronizing signal source is connected to theparent station A, the assumption in this embodiment being that theexternal synchronizing signal source EXT is disconnected. The normalstate is the same as the one shown in FIG. 22. The first stage of adisconnection of EXT is illustrated in FIG. 37. The station A served bythe external synchronizing signal source EXT detects an absence of theflag bit S supplied from the external synchronizing signal source EXTand selects the internal synchronizing signal source having the nexthighest priority according to the table T-A, while at the same timechanging the flag bit data, which had been transmitted to theneighboring stations B and D which stations had received thesynchronizing signal from the station A, from S to *S.

FIG. 38 illustrates the second stage that follows the stage shown inFIG. 37. The station D detects *S on the line connected to the station Aand checks, in accordance with the table T-D, the line connected to thestation C to the E direction, which line has the next highest priority.Since *S is again detected, the internal synchronizing signal INT isthen put to use, while at the same time the flag bit supplied to theline connected to the station C is changed from S to *S. The station Balso detects *S on the line connected to the station A and checks, inaccordance with the table T-B, the line connected to the station C tothe W direction, which line has the next highest priority. Since S isdetected, the line connected to the station C is put to use as thesynchronizing signal source, while at the same time the flag bit datasupplied to the line connected to the station C is changed from S to *Sand the flag bit data supplied to the line connected to the station A ischanged from *S to S.

FIG. 39 illustrates the third stage that follows the stage shown in FIG.38. The station C, detecting *S on the line connected to the station Dto the W direction, which line has the highest priority according to thetable T-C, checks the flag bit data from the station B to the Edirection because the line connected thereto has the next highestpriority. Since *S is detected again, the internal synchronizing signalsource is put to use, while at the same time the flag bit data suppliedto the line connected to the station B is changed from S to *S.

FIG. 40 illustrates the fourth stage that follows the stage shown inFIG. 39. The station B detects *S on the line connected to the station Cand changes over to the internal synchronizing signal source, while atthe same time changing the flag bit data supplied to the line connectedto the station A, from S to *S. That is, the station B changes over fromthe line (E) to the line (W) in a brief moment, but INT is put to use inno time, thereby putting the whole network temporarily in a stablestate.

A description will next be given of how the channel for transmitting thesynchronizing signal to the stations is recovered when the failure isremoved.

FIG. 41 illustrates the first stage of a recovery. The station A detectsS from the external synchronizing signal source EXT and selects EXT,while at the same time changing the flag bit data supplied to theneighboring stations B and D, from *S to S.

FIG. 42 illustrates the second stage of recovery that follows the stageshown in FIG. 51. The station D, which constantly checks the state ofthe synchronizing signal sources having higher priority than thecurrently used source, detects S on the line connected to the station Ato the W direction and selects the same line in accordance with thetable T-D, while at the same time changing the flag bit data supplied tothe station C, from *S to S. The station B, which constantly checks thestate of the synchronizing signal sources having higher priority thanthe currently used source, detects S on the line connected to thestation A to the E direction and selects the same line in accordancewith the table T-B, while at the same time changing the flag bit datasupplied to the station C, from *S to S. As a result of this, thestation C detects S on the lines to both directions, i.e., the E and Wdirections, and selects the line (W) because it has the highestpriority. This way, the initial normal state is regained.

FIG. 43 shows the stations A-D in a ring mode arrangement and thedetermination processes 1 and 2 carried out by the first determinationportion 14 and the second determination portion 16 in the stations. Inthe figure, "◯" indicates a normal operation (where no disconnectiontakes place), and "X" indicates a detection of a disconnection. P1, P2and P3 signify the order of priority for selection of a synchronizingsignal source. Holdover means a runaway state. The configuration of eachstation is the same as described with reference to FIG. 20.

For example, the determination is processed as follows in the firstdetermination portion 14 of the station A. The order of priority forselection of a synchronizing signal source is such that the highestpriority is given to the external synchronizing signal source EXT,followed by the internal synchronizing signal source 26. When theexternal synchronizing signal source EXT is operating normally, it isselected as the synchronizing signal source. When the externalsynchronizing signal source is disabled, the internal synchronizingsource 26 becomes the candidate for the synchronizing signal source. If,in this instance, the internal synchronizing signal source 26 isoperating normally, it is selected as the synchronizing signal source.If the internal synchronizing signal source 26 is disconnected, arunaway state sets in.

The determination is processed as follows in the second determinationportion 16 of the station A. When the external synchronizing signalsource EXT is selected, the transmitted flag bit data, supplied to thelines to the E and W directions, are set to be S. When the internalsynchronizing signal source 26 is selected or when a runaway state hasset in, the transmitted flag bit data supplied to the lines to bothdirections are set to be *S.

The first determination portion 14 and the second determination portion16 of the station B operate as shown in the same figure. The order ofpriority for selection of a synchronizing signal source for the stationB is such that the highest priority is given to the line to the Edirection, followed by the line to the W direction and the internalsynchronizing signal source 26, in the stated order. When the line tothe E direction is operating normally and the flag bit data receivedfrom the line to the E direction is S, the line to the E direction isselected as the synchronizing signal source. When the flag bit datasupplied from the line to the E direction is *S and the flag bit datasupplied from the line to the W direction is S, the line to the Wdirection is selected. When the flag bit data supplied from the lines tothe E and W directions are *S and *S, respectively, under the conditionthat the internal synchronizing signal source 26 is operating normally,the internal synchronizing signal source 26 is selected. When the lineto the E direction is operating normally and the internal synchronizingsignal source 26 is disconnected, a holdover sets in. When the flag bitdata received from the line to the W direction is S under the conditionthat the line to the E direction is disconnected and the line to the Wdirection is operating normally, the line to the W direction is selectedas the synchronizing signal source. When, in this instance, the flag bitdata received from the line to the W direction is *S, the internalsynchronizing signal source 26 is selected. When the flag bit datareceived from the line to the W direction is *S under the condition thatonly the line to the W direction is operating normally, a holdover setsin. When all of the lines are disconnected and only the internalsynchronizing signal source 26 is operating normally, the internalsynchronizing signal source 26 is selected. When all of thesynchronizing signal sources are disconnected or unavailable, a holdoversets in.

The second determination portion 16 of the station B operates asfollows. When the line to the E direction is selected, the flag bit datasupplied from the lines to the E and W directions are set to be *S andS, respectively. When the line to the W direction is selected, the flagbit data supplied from the lines to the E and W directions are set to beS and *S, respectively. When the internal synchronizing signal source 26is selected or when a holdover has set in, the transmitted flag bit datasupplied to the lines to the E and W directions are set to be *S and *S.

The first determination portion 14 and the second determination portion16 of the stations C and D operate as shown in the same figure. Theorder of priority for selection of a synchronizing signal source for thestations C and D is such that the line to the W direction is given thehighest priority, followed by the line to the E direction and theinternal synchronizing signal source 26, in the stated order. When theflag bit data supplied from the line to the W direction is S under thecondition that the line to the W direction is operating normally, theline to the W direction is selected as the synchronizing signal source.When the flag bit data supplied from the line to the E direction is Sunder the condition that both lines to the E and W directions areoperating normally, the line to the E direction is selected. The flagbit data supplied from the lines to the E and W directions are both *Sunder the condition that the line to the W direction and the internalsynchronizing signal source 26 are operating normally, the internalsynchronizing signal source 26 is selected. When the flag bit datareceived via the line to the E direction is S under the condition thatthe line to the W direction is disconnected and the line to the Edirection is operating normally, the line to the E direction isselected. Further, when the internal synchronizing signal source 26 isoperating normally and the flag bit data supplied from the line to the Edirection is *S, the internal synchronizing signal source is selected.When the flag bit data supplied from the line to the E direction is *Sunder the condition that the internal synchronizing signal source 26 isdisconnected, a runaway state sets in. When only the internalsynchronizing signal source 26 is operating normally, it is selected.

The second determination portion 16 of the stations C and D operates asshown in the same figure. When the line to the W direction is selected,the flag bit data supplied to the lines to the E and W directions areset to be S and *S, respectively. When the line to the E direction isselected, the flag bit data supplied to the lines to the E and Wdirections are set to be *S and S, respectively. When the internalsynchronizing signal source 26 is selected or when a runaway state hasset in, the flag bit data supplied to both lines to the E and Wdirections are set to be *S.

The flag bit data is defined such that it occupies a fixed position ineach frame (for example, the aforementioned reserved bit Z1).Accordingly, each station separates the frame multiplexed line signalinto frames so as to detect the flag bit data. Moreover, the flag bitdata contains a predetermined code consisting, for example, of only"1's" or of only "0's". It is desirable to employ a decision by majoritybetween "0's" and "1's" in order to make the network be tolerant of acertain degree of data error. For example, consider a case where a flagbit data occupies an information amount of 1 byte (8 bits) in one frame(for example, 19,440 kbps). Assuming that the flag bit data S and *S aredefined such that the former consists of only "1's" and the latterconsists of only "0's" and that a signal error involves a flag bit dataerror, a certain degree of tolerance to such an error can be achieved byconfiguring the process of detection as follows (an error having anerror rate of about 1.5×10⁻⁴ can yield a generally unaffecteddetermination).

    ______________________________________                                        flag bit data    detection   error rate                                       ______________________________________                                        8 bits of "1"    S           0                                                7 bits of "1", 1 bit of "0"                                                                    S           5.1 × 10.sup.-5                            6 bits of "1", 2 bits of "0"                                                                   S           1.0 × 10.sup.-4                            5 bits of "1", 3 bits of "0"                                                                   S           1.5 × 10.sup.-4                            4 bits of "1", 4 bits of "0"                                                                   indeterminable                                                                            2.1 × 10.sup.-4                            ______________________________________                                    

In the above-described embodiments, some of the stations allow theirinternal synchronizing signal sources to be used temporarily duringtransient stages in which the switching to alternative synchronizingsignal sources is proceeding. Even under a normal operation, theseinternal synchronizing signal sources maintain a high-precisionoscillation in sync with the first external synchronizing signal sourceEXT (P). The above-mentioned process of switching to alternativesynchronizing signal sources is completed in an extremely short periodof time. Accordingly, the phase shift of the oscillation during theperiod of switching to alternative synchronizing signal source isnegligibly small, thus ensuring quick and reliable switching toalternative synchronizing signal sources.

While only one external synchronizing signal source is assumed to beincluded in the above-described third and fourth embodiments of a ringmode arrangement, it is obvious that the present invention can beapplied to the case where a plurality of synchronizing signal sourcesare provided.

While the configuration of communication network employed in theabove-described embodiments are linear mode arrangement and a ring modearrangement, it is obvious that the present invention can be applied toa more complex network.

INDUSTRIAL APPLICATION

As is evident from the foregoing description, the present inventionenables constructing, with a very simple configuration, a digitalcommunication network using a plurality of synchronizing signal sourcesand makes it possible to quickly switch to reserved synchronizing signalsources upon an occurrence of a failure in the currently usedsynchronizing signal source and to provide a communication networkhaving remarkably improved reliability.

We claim:
 1. A method of switching from one synchronizing signal sourceto another in a synchronous data communication network including aplurality of stations with at least one child station and at least oneparent station connected to each other via lines, and a plurality ofsynchronizing signal sources provided for said stations, the methodcomprising, in combination, steps of:(a) setting, in a signaltransmitted via said lines and including synchronizing informationrelated to one of said plurality of synchronized signal sources, andflag bit data indicative of whether or not a timing of said one of theplurality of synchronizing signal sources related to said synchronizinginformation transmitted via said lines can be used as a synchronizingsignal source on each station which receives said informationtransmitted via said lines; (b) referring each station, from among saidplurality of stations on the basis of said flag bit data, to a tableprovided in each said station for specifying an order of priority forselection of a synchronizing signal source, upon occurrence of apredetermined event in any of said plurality of synchronizing signalsources, and for selecting a synchronizing signal source; and (c)switching each station from the currently selected synchronizing signalsource to the synchronizing signal source selected in said step (b). 2.The method of switching as claimed in claim 1, wherein,said plurality ofsynchronizing signal sources include at least one external synchronizingsignal source and a plurality of internal synchronizing signal sources,a table for a parent station using the external synchronizing signalsource during a normal operation specifies that said externalsynchronizing signal source be given highest priority and an internalsynchronizing signal source from among said plurality of internalsynchronizing signal sources be given lowest priority, and a table forchild stations receiving said synchronizing signal from other stationsspecifies that the station that serves as the source of thesynchronizing signal during a normal operation be given highest priorityand the station that serves as the source synchronizing signal when afailure occurs be given next highest order of priority, and also that,in the case that said child stations include internal synchronizingsignal sources, said internal synchronizing signal source be given thelowest order of priority.
 3. The method of switching as claimed in claim2, further comprising steps of:(d) sending from said parent station,during a normal operation, a first flag bit data S to all neighboringstations to inform neighboring stations that the synchronizing signalfrom said parent station can be used (e) sending from said parentstation when a failure occurs in said external synchronizing signalsource, a second flag bit data *S to inform neighboring stations thatthe synchronizing signal from said parent station cannot be used.
 4. Themethod of switching as claimed in claim 3, further comprising stepsof:(f) having each station select a selected station to receive thesynchronizing signal from, from among the neighboring stations, theselection being done in accordance with the table corresponding to saideach station when the first flag bit data is received from a neighboringstation, (g) having said each station send out the second flag bit datato said selected station, and (h) having said each station send thefirst flag bit data to a neighboring station other than said selectedstation.
 5. The method of switching as claimed in claim 4, furthercomprising steps of:(i) having each station neglect, when the stationreceives the second flag bit data from the neighboring station, theneighboring station originating the second flag bit data; and (j)operating the child stations by using the internal synchronizing signalsource to send out the second flag bit data to their neighboringstation.
 6. The method of switching as claimed in claim 1, wherein saidplurality of stations connected via the lines are in a linear modearrangement.
 7. The method of switching as claimed in claim 1, whereinsaid plurality of stations connected via the lines are in a ring modearrangement.
 8. The method of switching as claimed in claim 1, whereinsaid synchronizing signal sources include the external synchronizingsignal source connected to a plurality of stations.
 9. The method ofswitching as claimed in claim 1, wherein said synchronizing signalsources include an external synchronizing signal source connected to onestation having an internal synchronizing signal source, andsaid methodof switching further comprises a step of switching to said internalsynchronizing signal source when a failure occurs in said externalsynchronizing signal source.
 10. A communication apparatus used in asynchronous data communication network having a plurality ofsynchronizing signal sources, including at least one parentsynchronizing signal source and at least one child synchronizing signalsource, the apparatus comprising:first means for receiving flag bit dataindicating whether or not a timing of said one of the plurality ofsynchronizing signal sources related to information in a synchronizingsignal transmitted via a line and including information related to oneof said plurality of synchronizing signal sources can be used as asynchronizing signal source on each station which receives saidinformation, said synchronizing information being included in the signaltransmitted via the line; second means for specifying an order ofpriority for selection of a synchronizing signal source; third means forreferring, upon occurrence of a predetermined event in any of saidplurality of synchronizing signal sources, to the order of priority forselection of synchronizing signal source, the order being specified bysaid second means, and for selecting, on the basis of the flag bit data,a synchronizing signal source; and fourth means for sending, incorrespondence to the synchronizing signal source selected by the thirdmeans, flag bit data, indicating whether or not a timing of said one ofthe plurality of synchronizing signal sources related to synchronizinginformation from the communication apparatus originating the flag bitdata can be used as a synchronizing signal source on each stations whichreceives said signal transmitted via said lines, to neighboringcommunication apparatuses.
 11. The communication apparatus as claimed inclaim 10, wherein said plurality of synchronizing signal sources includeat least one external synchronizing signal source, andthe second meansof said communication apparatus specifies, when said communicationapparatus functions as a parent station using the external synchronizingsignal source during a normal operation, that the external synchronizingsignal source be given highest priority and the internal synchronizingsignal source be given lowest priority.
 12. The communication apparatusas claimed in claim 10, wherein the second means of said communicationapparatus specifies, when said communication apparatus functions as achild stations receiving the synchronizing signal from another station,that the station that serves as the source of the synchronizing signalduring a normal operation be given the highest priority and station thatserves as the source of the synchronizing signal when a failure occursbe given next highest priority.
 13. The communication apparatus asclaimed in claim 11, wherein said communication apparatus furthercomprises one internal synchronizing signal source, andthe second meansof said communication apparatus specifies that said internalsynchronizing signal source be given the lowest priority.
 14. Thecommunication apparatuses in claim 11, wherein the fourth means of saidcommunication apparatus comprises:fifth means for having saidcommunication apparatus functioning as the parent station during anormal operation to send, to neighboring stations, the first flag bitdata S indicative of availability to inform all of said neighboringstations that the synchronizing signal from said parent station can beused; and sixth means for having said parent station send, when afailure occurs in said external synchronizing signal source, the secondflag bit data *S indicative of the non-availability to neighboringstations to inform all neighboring stations that the synchronizingsignal from said parent station cannot be used.
 15. The communicationapparatus as claimed in claim 10, wherein said third means of saidcommunication apparatus includes fifth means for selecting synchronizingsignal sources, other than the synchronizing signal source indicated asnot usable by said flag bit data, in accordance with the order ofpriority specified by said second means.
 16. The communication apparatusas claimed in claim 11 wherein the predetermined event is a failure. 17.The communication apparatus as claimed in claim 10 wherein thepredetermined event is a failure.